This invention relates in general to magnetoresistive (MR) read sensors or heads for magnetic recording systems, and more particularly to such sensors that operate in the xe2x80x9ccurrent-perpendicular-to-the-planexe2x80x9d or CPP mode.
In certain types of MR read sensors or heads for magnetic recording systems, the sense current passes perpendicularly through the planes of the layers making up the sensor. Such sensors are called xe2x80x9ccurrent-perpendicular-to-the-planexe2x80x9d or CPP sensors. CPP sensors are distinguished from xe2x80x9cspin-valvexe2x80x9d type MR sensors widely used in commercially available magnetic recording disk drives because spin-valve sensors operate with the sense xe2x80x9ccurrent-in-the-planexe2x80x9d of the sensor layers, or in CIP mode.
One type of CPP sensor is a magnetic tunnel junction (MTJ) sensor comprised of two ferromagnetic layers separated by a thin insulating tunnel barrier layer and based on the phenomenon of spin-polarized electron tunneling. The response of a MTJ sensor is determined by measuring the resistance of the MTJ when a sense current is passed perpendicularly through the MTJ from one ferromagnetic layer to the other. The probability of tunneling of charge carriers across the insulating tunnel barrier layer depends on the relative alignment of the magnetic moments (magnetization directions) of the two ferromagnetic layers. In addition to MTJ sensors, giant magnetoresistive (GMR) type of MR sensors have also been proposed to operate in the CPP mode, as described by Rottmayer and Zhu, xe2x80x9cA new design for an ultra-high density magnetic recording head using a GMR sensor in the CPP modexe2x80x9d, IEEE Transactions on Magnetics, Vol 31, Issue 6, Part: 1, November 1995, pp. 2597-2599; and in U.S. Pat. No. 5,883,763.
One of the problems with CPP MTJ and GMR sensors is the ability to generate an output signal that is both stable and linear with the magnetic field strength from the recorded medium. If some means is not used to stabilize the sensing ferromagnetic layer in the CPP sensor, then magnetic instabilities and hysteresis (Barkhausen noise) will degrade the signal to noise performance of the sensor. The problem of sensor stabilization using a conventional tail stabilization approach is especially difficult in the case of a CPP sensor, like an MTJ MR read head, because the sense current passes perpendicularly through the ferromagnetic layers and the tunnel barrier layer, and thus any metallic materials used in the tails to stabilize the sensing ferromagnetic layer will short circuit the electrical resistance of the MTJ if they come in contact with the ferromagnetic layers.
IBM""s U.S. Pat. No. 6,023,395 describes an MTJ MR read head that has a biasing ferromagnetic layer magnetostatically coupled with the sensing ferromagnetic layer of the MTJ to provide longitudinal bias to the sensing ferromagnetic layer. As shown in FIG. 1, this MTJ MR head is a sensor structure made up of a stack of layers formed between a bottom shield 10 and a top shield 12, the shields being typically formed of relatively thick highly magnetically permeable material, such as permalloy (Ni100xe2x88x92x Fex, where x is approximately 19). The shields 10, 12 have generally planar surfaces spaced apart by a gap 53. The gap material 50, 52 on the sides of the sensor structure is an insulating material, typically an oxide such as alumina (Al2O3). The layers in the stack are a bottom electrical lead 20, the MTJ sensor 30, the longitudinal bias stack 40, and top electrical lead 22. The MTJ sensor 30 is made up of an antiferromagnetic layer 32, a fixed ferromagnetic layer 34 exchange biased with the antiferromagnetic layer 32 so that its magnetic moment cannot rotate in the presence of an applied magnetic field, an insulating tunnel barrier layer 36 in contact with the fixed ferromagnetic layer 34, and a sensing or xe2x80x9cfreexe2x80x9d ferromagnetic layer 38 in contact with the tunnel barrier layer 36 and whose magnetic moment is free to rotate in the presence of an applied magnetic field. The longitudinal bias stack 40 includes a nonmagnetic electrically conductive spacer layer 42, a biasing ferromagnetic layer 44 that has its magnetic moment aligned generally within the plane of the device and is separated from the ferromagnetic layer 38 by the spacer layer 42, and optionally an antiferromagnetic layer 46 exchange coupled to the biasing ferromagnetic layer 44. The self field or demagnetizing field from the biasing ferromagnetic layer 44 magnetostatically couples with the edges of the sensing ferromagnetic layer 38 to stabilize its magnetic moment, and, to linearize the output of the device. The electrically conductive spacer layer 42 prevents direct exchange coupling between the biasing ferromagnetic layer 44 and the sensing ferromagnetic layer 38 in the MTJ sensor 30 and allows sense current to flow perpendicularly through the layers in the stack between the two leads 20, 22.
The width of the data tracks of the recorded media is determined by the trackwidth (TW) of the MR sensor, as shown in FIG. 1. The shielding geometry provided by shields 10, 12 of the MR sensor attenuates the flux coming from adjacent magnetic transitions of the recorded media along the downtrack direction (perpendicular to the layers in the stack) and therefore enhances the sensor""s linear resolution. However, it has been discovered as part of the development of the present invention that for very small trackwidths this shielding geometry does not provide adequate suppression of side reading caused by flux coming from adjacent tracks.
What is needed is a CPP sensor with in-stack longitudinal biasing that does not suffer from side reading of adjacent data tracks.
The invention is a CPP magnetoresistive sensor or read head with a magnetic shield geometry that covers the side walls of the sensor structure to prevent side reading caused by magnetic flux entering from adjacent data tracks. The shield geometry includes a top shield that has nearly vertical portions generally parallel to the side walls of the sensor structure, horizontal side portions over the portions of the bottom shield on either side of the sensor structure, and a top horizontal portion over the top trackwidth region of the sensor. The insulating gap material that separates the bottom and top shields is in contact with the horizontal portions of the bottom shield and the side walls of the sensor structure. Because of the nearly vertical portions of the top shield, the distance from the bottom shield to the sensing layer of the sensor structure is greater than the gap thickness outside the sensor structure so that magnetic flux is generally prevented from entering the sides of the sensing layer.
For a fuller understanding of the nature and advantages of the present invention, reference should be made to the following detailed description taken together with the accompanying figures